This invention relates to a catalyst for producing higher carbon number hydrocarbons from low carbon number hydrocarbons, such as methane. The catalyst comprises a porous support having dispersed thereon rhenium and a metal selected from the group consisting of iron, cobalt, vanadium, manganese, molybdenum, tungsten and mixtures thereof. An example of the porous support is zeolite ZSM-5. This invention also relates to a process for preparing the catalyst and a process for producing higher carbon number hydrocarbons using the catalyst. The process for preparing higher carbon number hydrocarbons comprises contacting low carbon number aliphatic hydrocarbons with a catalyst in the presence of CO or CO2 at conversion conditions to produce the higher carbon number hydrocarbons.
It is well known to produce aromatic compounds such as benzene, toluene and xylenes from petroleum naphtha streams. Attempts have also been made to produce useful aromatic compounds from low molecular weight aliphatic compounds by, for example, the pyrolysis of natural gas, acetylene and other gases. However, this technique produces benzene and other useful aromatic compounds in very low yields while producing large amounts of tar, insoluble carbon residue and high molecular weight aromatic compounds, all of which are of little commercial use. Specifically, in the pyrolysis of methane and acetylene, the reaction is carried out at a temperature of about 1,000xc2x0 C. or higher with a conversion rate of only a few percent and a selectivity to naphthalenes of less than 1%. Consequently, this method has little practical application.
There are reports in the art of processes for converting natural gas into aromatic compounds. For example, U.S. Pat. No. 5,288,935 discloses a process for producing liquid hydrocarbons from natural gas, in which natural gas is first separated into a methane rich fraction and a C2+ fraction, the methane is then selectively oxidized with oxygen, the effluent from the selective oxidation is then mixed with a part of the C2+ fraction and pyrolyzing the resulting mixture to obtain an aromatic product. The final step is carried out at a temperature of about 300xc2x0 C. to about 750xc2x0 C. in the presence of an aromatizing catalyst consisting essentially of a zeolite, gallium, at least one metal from the Group VIII metals and rhenium and at least one additional metal selected from the group consisting of: tin, germanium, lead, indium, thallium, copper, gold, nickel, iron, chromium, molybdenum and tungsten; an alkaline metal or alkaline earth metal and an aluminum matrix.
It is also known that the dehydrocondensation of methane with CO or CO2 to form benzene and naphthalene can be carried out using a molybdenum/HZSM-5 or iron/cobalt modified Mo/HZSM-5. S. Liu, Q. Dong, R. Ohonishi and M. Ichikawa, Chem. Commun. (1998), p. 1217-1218, and S. Liu, L. Wang, Q. Dong, R. Ohonishi, and M. Ichikawa, Stud. Surf. Sci. Catal., Vol. 119, p. 241-246. In contrast to this art, applicants have developed a novel catalyst which comprises rhenium on a porous support such as a zeolite and which optionally can contain other metals such as iron, cobalt, platinum and molybdenum. It has been found that the catalysts of the present invention have higher activities for converting methane to benzene and also have a higher selectivity for the higher carbon number hydrocarbon products such as benzene, toluene and xylene and ethane and ethylene.
As stated, the present invention relates to a catalyst for converting low carbon number aliphatic hydrocarbons to higher carbon number hydrocarbons, a process for preparing the catalyst, and a process for converting low carbon number aliphatic hydrocarbons to higher carbon number hydrocarbons. Accordingly, one embodiment of the invention is a catalyst for converting low carbon number aliphatic hydrocarbons to higher carbon number hydrocarbons comprising a porous support having dispersed thereon rhenium and a promoter metal selected from the group consisting of iron, cobalt, vanadium, gallium, zinc, chromium, manganese, molybdenum, tungsten and mixtures thereof. Another embodiment of the invention is a process for preparing the catalyst described above, the process comprising impregnating the support with a rhenium compound and a promoter metal compound, calcining the impregnated support at calcination conditions to give a calcined product and treating the calcined product with hydrogen and methane at treatment conditions to give the catalyst.
Yet another embodiment of the invention is a process for converting low carbon number aliphatic hydrocarbons to higher carbon number hydrocarbons comprising contacting the low carbon number aliphatic hydrocarbons at conditions to give the higher carbon number hydrocarbons. These and other objects and embodiments will become more apparent after a detailed description of the invention.
One aspect of the current invention is a novel catalyst for carrying out dehydrocondensation of methane. One essential element of this catalyst is a porous inorganic oxide support. This support can be chosen from a wide variety of supports which have a high surface area, porous structure and are preferably acidic. Examples of these supports include zeolites, non-zeolitic molecular sieves, silica, alumina and mixtures thereof. The zeolites which can be used as the support include any of those which have a SiO2/Al2O3 ratio between 1 and 8,000 and preferably in the range of about 10 to about 100. The zeolites have channels or pores of about 0.5 to about 10 nm. The porous zeolite may contain Al, Ti, Zr, Ga, Zn, V, Cr and mixtures thereof, i.e., a metallosilicate. The surface area of these materials is preferably in the range of about 100 to about 1,000 m2/g. Specific examples of the molecular sieves which can be used as the support for the present catalyst include zeolite Y, zeolite X, mordenite, ZSM-5, ALPO-5, VPI-5, FSM-16, MCM-22 and MCM-41. The inorganic support may be used in any desirable form such as powder, pellets, extrudates, spheres, and irregularly shaped particles.
Having formed the support into a desired shape, the next step in preparing the catalyst is to disperse rhenium and a promoter metal onto the support. The promoter metal which can be present is selected from the group consisting of iron, cobalt, vanadium, manganese, gallium, zinc, chromium, tungsten, molybdenum and mixtures thereof.
The rhenium and promoter metal can be dispersed on the porous support by means well known in the art such as impregnation, spray-drying, ion-exchange, vapor deposition, etc. Impregnation of the support with the rhenium and promoter metal can be carried out using decomposable compounds of rhenium and the promoter metals. By decomposable compound is meant that upon heating the compound decomposes to give the corresponding metal or metal oxides. Example of the rhenium compounds which can be used are NH4ReO4, CH3ReO3, Re2O7, ReO3, ReS2, Re2S7, Re2(CO)10, NH4Re(CO)5, MnRe(CO)10, Co[Re(CO)5]2, M2[Re3H3(CO)10], Re3H(CO)14, M2[Re4H4(CO)12], M2[Re4(CO)16](Mxe2x95x90NEt4, NBU4, Li, Na, K and NH4), ReCl3, ReC15, [Re2X3(CO)7] (Xxe2x95x90I, Cl, Br) [Re(CO)6][Re2F11]; Mxe2x80x2[Re6S9Xxe2x80x26](Mxe2x80x2xe2x95x90NBu4, NEt4; Xxe2x80x2xe2x95x90Cl, Br); Mxe2x80x32[ReS8(Py)2Xxe2x80x34(Mxe2x80x3xe2x95x90NBu4, Xxe2x80x3xe2x95x90Cl, Br, py=pyridine).
Examples of the compounds of iron, cobalt, vanadium, manganese and molybdenum which can be used include the halides, nitrates, sulfates, phosphates, carbonates, acetates and oxalates. Other examples of molybdenum compounds which can be used include ammonium paramolybdate, 12-phosphomolybdic acid, 12-silicomolybdic acid and 12-phosphomolybdic vanadic acid, MoS3, Mo(CO)6, [Mo3(CH3C)(O)(CH3COO)9]X(Xxe2x95x90Cl, Br and I) and (Mo2(CH3COO)6 and mixtures thereof. The rhenium and promoter metal may be deposited on the support by vapor deposition, ion-exchange or impregnation from a common aqueous or organic solvent solution, sequentially in any order. A preferred method comprises first depositing rhenium on the support and then depositing at least one promoter metal. The preferred method of depositing rhenium and the promoter metal is by impregnation. It should be pointed out that when zeolites or molecular sieves are the supports both deposition and ion-exchange of the metals can occur. Therefore, in the present context, impregnation will encompass ion-exchange as well as conventional impregnation. When the rhenium and promoter metal are impregnated sequentially, after the rhenium has been impregnated, the resulting impregnated support is dried and calcined at a temperature of about 50xc2x0 C. to about 800xc2x0 C. to give a calcined rhenium support and then impregnation is carried out with a solution containing at least one metal compound. After this second impregnation, or after the rheniun and metal compounds have been co-impregnated, the support is calcined at a temperature of about 50xc2x0 C. to about 800xc2x0 C. for a time of about 0.5 hr. to about 100 hr. Next, the calcined catalyst is activated by treating the catalyst with a hydrogen/ and/or methane treatment gas at a temperature of about 100xc2x0 C. to about 800xc2x0 C. for a time of about 0.5 hr. to about 100 hr. The amount of rhenium and promoter metal which is dispersed in the final catalyst can vary considerably, but usually for the rhenium varies from about 0.001 wt. % to about 50 wt. % of the support and for the promoter metal varies from about 0.001 wt. % to about 50 wt. % of the support.
Having obtained the catalyst of the invention, it can now be used in a process for converting low carbon number aliphatic hydrocarbons to higher number hydrocarbons. More specifically, the process if a dehydrocondensation process in which aliphatic compounds such as methane are converted to aromatic compounds such as benzene and naphthalene plus ethylene or ethane. Since dehydrogenation is part of the reaction, hydrogen is produced during the process. By low carbon number aliphatic hydrocarbons is meant any aliphatic hydrocarbon having from 1 to 4 carbon atoms. The process works especially well with methane. Therefore, the feedstream which can be used in the process of the invention, can be any feedstream which contains at least 5% methane and preferably at least 20% methane. Provided the gas stream contains at least the above amounts of methane, the stream can also contain C2-C4 saturated and unsaturated hydrocarbons such as ethane, ethylene, acetylene, propane, propylene, n-butane, isobutane, butene, isobutene, etc.
The gas stream is contacted with the catalyst at conversion conditions either in a batch mode or a continuous flow mode, with continuous flow being preferred. In the continuous flow mode, the catalyst can be present as a fixed bed, moving bed, or fluidized bed. The process is carried out by contacting the feedstream in the absence of oxygen at a temperature of about 300xc2x0 C. to about 1000xc2x0 C. and preferably at a temperature of about 450xc2x0 C. to about 900xc2x0 C., a pressure of about 10 kPa to about 1000 kPa and preferably from about 100 to about 1000 kPa and a weight hourly space velocity in the range of about 100 to about 200,000 hrxe2x88x921. It is also preferred that the reaction zone contain CO, CO2 or mixtures thereof, component in order to increase the selectivity to benzene and other aromatic compounds. The CO, CO2 or mixtures thereof to methane ratio varies from about 0.001 to about 0.5 and preferably from about 0.01 to about 0.3. The effluent from the reaction zone can be separated by conventional means and the unreacted feed gas recycled to the reaction zone.
In order to more fully illustrate the invention, the following examples are set forth. It is to be understood that the examples are only by way of illustration and are not intended as an undue limitation on the broad scope of the invention as set forth in the appended claims.